GA-A25180

CONTROL IMPROVEMENTS FOR THEDIII-D CRYO SYSTEM

byP.S. MAUZEY, K.L. HOLTROP, and P.M. ANDERSON

SEPTEMBER 2005

QTYUIOP

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability orresponsibility for the accuracy, completeness, or usefulness of any information, apparatus,product, or process disclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by trade name,trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of the UnitedStates Government or any agency thereof.

GA-A25180

CONTROL IMPROVEMENTS FOR THEDIII-D CRYO SYSTEM

byP.S. MAUZEY, K.L. HOLTROP, and P.M. ANDERSON

This is a preprint of a paper to be presented at the 21st

IEEE/NPSS Symposium on Fusion Engineering 2005,Knoxville, Tennessee, September 26-29, 2005, and to beprinted in the Proceedings.

Work supported bythe U.S. Department of Energy

under DE-FC02-04ER54698

GENERAL ATOMICS PROJECT 30200SEPTEMBER 2005

QTYUIOP

CONTROL IMPROVEMENTS FOR THE DIII-D CRYO SYSTEM P.S. Mauzey, et al.

GENERAL ATOMICS REPORT A25180 1

Control Improvements for the DIII-D Cryo System*

P.S. Mauzey, K.L. Holtrop, P.M. Anderson

General Atomics, San Diego, California

Abstract�The cryogenic system at General Atomics� DIII-D

Tokamak fusion facility provides cryo-pumping for the fusion

vessel and neutral beams, cooling for the superconducting

magnets for electron cyclotron heating, and cooling for the

deuterium pellet injector. The cryo system, built in the 1980s, has

been upgraded to integrate control using one programmable logic

controller (PLC). This integration of control has provided the

ability to make improvements to the control of the cryo system.

I. INTRODUCTION

A. Cryo System Description

The cryogenic system supporting General Atomics� DIII-DTokamak (Fig. 1) provides cryo-pumping of gases in theDIII-D vessel, neutral beamlines, and gyrotron magnets usingliquid helium at a temperature of 4.6 K. All closed loop liquidhelium users are arranged as seven parallel circuits for fourneutral beams and three in-vessel cryo pumps (Fig. 2). Each ofthese seven users requires about five grams per second of liquidhelium. The cryo system also provides once through batch-fillcooling for the heating gyrotrons� super conducting magnetsand cooling for the deuterium pellet injector.

DIII-D�s cryo system includes a two-turbine heliumliquefier, shown in Fig. 1. The helium system is supplied bytwo helium compressors producing 1.62 MPa (220 psig) heli-um gas at 115 grams per second maximum combined flow rate.Helium gas is stored at atmospheric temperature in four hori-

zontal cylindrical tanks. Liquefied helium is stored in a 3.8 m3

(1000 gal) storage dewar. The flow of liquid helium throughthe cryo system is driven by the difference in pressure betweenthe liquid helium storage dewar at 0.143 MPa (6 psig) and thehelium gas returning to the compressors, at about 0.105 MPa(0.6 psig).

The cryo system�s helium gas management system (GMS)(Fig. 2) is designed to maintain a constant pressure in the lowpressure helium gas returning from users, accomplished bymaintaining a constant inventory of helium gas in the system.When the pressure drops below the set point on the lowpressure side, helium gas from the helium gas storage tanksenters the low pressure side through a PLC-controlled �makevalve.� When the low pressure side rises above the set point,helium gas leaves the high pressure side, going to the heliumgas storage tanks through a PLC-controlled �dump valve,� untilthe system�s helium gas inventory is reduced to the point thatthe low pressure side is at the set point again. The pressure inthe high pressure (compressor discharge) side is maintained bya back-pressure-regulating bypass valve. This bypass valvedischarges excess high pressure helium to the low pressureside. Some flow through the bypass valve is normal andcontributes to system stability during transient events thatchange the amount of helium gas in the system�s inventorysuch as occurs during cryo pump defrost and cool down.

Liquid nitrogen is used for shielding liquid helium fromthermal (heat) radiation on long cryo lines, distribution valvecold boxes, cryostats, subcoolers, heat exchangers, cryo pumpsand cryo panels, and super conducting ECH magnets. Liquidnitrogen is also used for precooling helium gas entering thehelium liquefier.

Liquid nitrogen is bought as bulk liquid and stored in a

42 m3 (11,000 gal) storage tank at 0.446 MPa (40 psig). Allliquid nitrogen used in the cryo system ultimately is convertedto gas, and is vented to the atmosphere. The flow of liquidnitrogen through the cryo system is driven by the pressure inthe liquid nitrogen storage tank.

B. Cryo System Instrumentation and Control

Control and monitoring instrumentation for the cryo systemhas evolved over its 25 year life. Early 1980s technologyincluded pneumatic instrumentation to provide input and outputsignals for valve controllers and analog instruments. This pneu-matic instrumentation, along with hand toggle switches, indi-cator lights, electrical relays, solenoid valves, digital readoutsand dial gauges were mounted in instrument panels, installed inrows of control cabinets. As upgrades and additions to the cryosystem were made, control and monitoring instrumentationevolved into a combination of the original instrument panelsand three PLCs (programmable logic controllers). Accuratemass flow transmitters were installed to measure gaseoushelium flow at the compressor discharge, GMS bypass, lique-fier inlet, return from the two upper cryo pumps and the lowercryo pump, return from the four neutral beams, and highpressure supply to the reliquefying heat exchangers. Over thelast four years, the control and monitoring of the cryo systemhas been integrated into one PLC [1], a Siemens SimanticTI555 PLC, with Ethernet links to replace slower RS-232 serialcommunications and 4-20 mA input and output analog signalsto replace older 3-15 psi pneumatic signals.

Figure 1. DIII-D cryo system�s liquefier, helium storage dewar, wet expanderand distribution boxes.

*Work supported by the U.S. Department of Energy under Contract No.

DE-FC02-04ER54698.

P.S. Mauzey, et al. CONTROL IMPROVEMENTS FOR THE DIII-D CRYO SYSTEM

2 GENERAL ATOMICS REPORT A25180

Figure 2. Block diagram of the DIII-D cryo system.

The integrated and faster control system allows real-timecontrol and monitoring of process variables from the operatorstation located in the cryo control room. The cryo operator�sstation consists of a personal computer with an instrumentinput/output database and a graphical user interface. In additionto control loop screens, status screens, alarm screens, helpscreens, mode control screens, and an event logging screen,about 30 process diagram screens have been developed,displaying cryo system process variables. Each variable can beviewed on a real-time trending chart. Charts with trending ofmultiple process variables have provided increasedunderstanding of the transients of two-phase cryogen flow, andhow they affect the cryo system. Understanding of the cryosystem�s processes along with integrated control using a singlePLC have lead to several improvements to the control of thecryo system.

II. CONTROL IMPROVEMENTS

A. Night/Weekend Mode

During normal physics operations, the cryo system runs atmaximum capacity, with both helium compressors fully loaded.By the end of each day�s experiments, the inventory of liquidhelium needs to be replenished. This was previously accom-plished by estimating the helium liquefier�s make rate to fill thestorage dewar by morning, and leaving the liquefier andcompressors to run all night or all weekend.

After integrating the control of the cryo system, PLC logicwas developed to create a �Night/Weekend Mode.� Overnightand weekend operation of the system is automatic without anoperator. Between physics operational days, the in-vessel cryopumps are shut down leaving the neutral beams to becontinuously cooled. At the end of daily operations, the cryooperator selects this mode at the operator�s screen. In thismode, the PLC checks interlocks, then starts ramping up theliquefier�s make rate to 100% while adjusting the loading of thesecondary helium compressor to maintain helium flow in thebypass valve. When the liquid helium storage dewar is full, thePLC ramps down the liquefier�s make rate to maintain aconstant level in the storage dewar and adjusts the loading ofthe secondary compressor. The PLC will shut down thesecondary compressor if enough flow is provided by theprimary compressor. This occurs during the night, as opposedto running the secondary compressor continuously.

In addition, an �evening checklist� screen was developed toprovide the cryo operator the ability to quickly and reliablycheck the status of the cryo system when leaving at the end ofthe day. This is a summary of important data normally shownon ten or more screens. This screen shows control modes on oroff, valves open or closed, and the values of several variablessuch as tank and dewar levels. The normal status of each ishighlighted in green; abnormal conditions are highlighted inred or yellow. This has been useful in reducing after-hour call-

CONTROL IMPROVEMENTS FOR THE DIII-D CRYO SYSTEM P.S. Mauzey, et al.

GENERAL ATOMICS REPORT A25180 3

ins due to operator errors, and in reducing the effort required tocheck the status of the cryo system.

B. Neutral Beam Cryo PLC Upgrades

The flow of liquid helium and liquid nitrogen through eachof the four neutral beam cryo panels is controlled by a valve atthe inlet of each cryo panel, eight control valves in all. Thecontrol valve actuators were converted from pneumatic toelectro-pneumatic actuators, to allow the cryo PLC to controlthe valves instead of the older pneumatic controllers. Thepneumatic controllers have performed reliably over the years,but the PLC allows integration of the control of these cryopanels with the rest of the cryo system.

The flow of liquid helium through a beamline cryo panel iscontrolled by maintaining a constant differential pressurebetween the inlet of the cryo panel and the two-phase outlet ofthe cryo panels. The flow of liquid nitrogen through a beamlinecryo panel is controlled by maintaining a constant gastemperature at the outlet of the cryo panels. Control of thehelium and nitrogen panels was previously accomplished bypneumatic controllers with adjustable setpoint and adjustableproportional, integral and differential control loop values. Byconverting these pneumatic controllers to PLC software controllogic, several control improvements were achieved:

1. When upsets in the neutral beam helium cryo panels occurdue to unusual heat loads on the cryo panels, temperaturesensors on the cryo panels provide a signal to the PLC of atemperature excursion, which increases the flow of heliumuntil the temperature is again cold and stable. Beforeimplementing, this unpredictable occurrence was difficultand time consuming to recover from manually.

2. When starting or stopping certain helium cryo pumps in theDIII-D vessel, certain other neutral beam cryo panels wereknown to experience reduction of liquid helium flow. Bytrend analysis, the flow reduction was determined to becaused by variations in the back-pressure of the helium gasreturning through a low pressure return line shared by theneutral beam and the cryo pump. Logic was implemented toautomatically increase the flow of helium to the affectedbeamlines, to pre-empt a loss of flow, during these changesto the cryo system.

3. Beamline cryo panels can be remotely cooled or warmedthrough a remote laptop from off-site during nights andweekends. This allows earlier starting of physics operationsafter a maintenance shutdown.

4. Regeneration (defrosting) of beamline cryo panels can beaccomplished automatically. Once implemented, this willreplace manual regeneration and save operator timerequired.

C. Cryo Pump Tuning

Each of the three cryo pumps in DIII-D (Fig. 3) providescryo-condensation of gases used in physics experiments,typically deuterium. When a cryo pump is used for a day�sexperiments, it is kept at about 4K during a tokamak shot bymaintaining a minimum He flow rate of about 6 grams persecond. At the end of each tokamak shot, the cryo pump isdefrosted by warming it to greater than 20 K. This is

accomplished by closing the liquid helium inlet valve andintroducing a small amount of room temperature helium gas fora short period of time. It is held at that condition until the endof glow discharge, about ten minutes, when it is then recooledby opening the inlet valve and allowing liquid helium to returnthe cryo pump to a cold, steady-state condition. This defrostprevents buildup of frost on the pumping surfaces. The totaldefrost time is about fifteen minutes.

It was observed that after each tokamak shot, a lack of flowin the GMS bypass valve caused a loss of stability in the gasmanagement system. When this occurred, interlocks intendedto protect the cryo system would shut down the cryo pumps,and shut down the helium liquefier. Using multi-variable real-time trending charts, it became clear that the solution topreventing shutdowns was to reduce surges in helium gasreturning from the cryo pumps. This was achieved byimplementing several changes to the control logic of the cryopumps:

1. During cryo pump recooling, helium gas leaving the cryopump was discharged directly into the low pressure side ofthe cryo system. This caused a surge of gas into the lowpressure side as liquid helium boiled as it cooled the cryopump. The logic was changed so that helium leaving thecryo pump during recool passed through the boiling heatexchanger before entering the low pressure side. The heatexchanger provides a cold thermal mass to reduce the surgein helium gas.

2. After the cryo pumps were defrosted, the flow of liquidhelium remained off until the end of glow discharge,typically lasting about ten minutes. Changes were made inthe logic to keep a small flow of liquid helium through thecryo pumps during glow discharge. This reduced the surgeof helium gas from the cryo pumps returning to the lowpressure side of the cryo system at the start of recool.

Figure 3. DIII-D�s three in-vessel cryo pumps. Photo of the model of DIII-D.

P.S. Mauzey, et al. CONTROL IMPROVEMENTS FOR THE DIII-D CRYO SYSTEM

4 GENERAL ATOMICS REPORT A25180

D. Cool Down Improvements

Cooldown of the cryo system nitrogen circuits wasoptimized to reduce the cool down time at the beginning ofphysics operations.

� The liquefier�s control valve for liquid nitrogen wasincreased from 35% to 100% open during cool down.

� The flow rate of liquid nitrogen for the cryo pump�scryostat shield was increased during cool down. Aparallel line with a PLC-controlled solenoid valve wasadded to provide extra flow.

� Logic was implemented to control the nitrogensubcoolers for the neutral beam cryo panel. Thesesubcoolers are used to increase the amount of liquidnitrogen in the two-phase flow through the cryo panels.However, they also increase the time required for cooldown. By adding a solenoid valve bypassing thesubcooler and a pressure relief valve at the outlet of thesubcooler, the PLC is used to bypass the subcoolerduring cooldown.

E. Remote Operation

The cryo system at DIII-D operates continuously duringoperating periods. Remote monitoring and control of the cryosystem from a laptop allows an operator to monitor andrespond from home to cryo system alarms occurring afterhours. When an alarm from the cryo system PLC notifiesGeneral Atomics� guard station, a security guard calls the cryooperator on-duty. The operator uses the laptop to identify thealarm, determine the cause, and take appropriate action. Theremote laptop is also used to start the cooling of cryo panelsand cryo pumps the evening before operations.

F. Monitoring of Nitrogen Use

The installation of nitrogen flow meters used for eachneutral beam lead to the optimized tuning of the nitrogencontrollers. These flow meters are accurate, temperature-compensated mass flow meters, connected to the PLC forobserving and recording flow data. In a typical beamline, theaverage flow rate of nitrogen was reduced from 77.8 to 50.0standard cubic centimeters per second (16.5 to 10.6 standardcubic feet per minute).

Mass flow meters are being implemented to measure theamount of liquid nitrogen used by each of the three in-vesselcryo pumps, the four neutral beam cryo panels, and the heliumliquefier, and the amount vaporized for use by laboratories atDIII-D. These flows, measured after the liquid nitrogen isvaporized, are then totaled by the cryo PLC and compared tothe drop in the level of the 11,000 gal liquid nitrogen tank, toprovide an accurate accounting of the liquid nitrogen usage atthe DIII-D facility. This accounting will be made continuouslyby the PLC.

III. RESULTS OF CONTROL IMPROVEMENTS

A. Power, Cost and Time Savings

The night/weekend mode shuts down the secondarycompressor during the night as opposed to running thesecondary compressor continuously, saving $17,000 per year inelectricity.

Tuning the control of nitrogen in the neutral beam cryopanels reduced the usage of liquid nitrogen, saving $20,000 peryear.

Reduced cool down time and the use of the remote lap haveeliminated a three-hour delay at the beginning of physicsoperating periods.

Monitoring of nitrogen usage resulted in the discovery andcorrection of a nitrogen gas leak, saving $25,000 per year.

B. After-Hour Call-Ins

Improvements to the cryo system control have made thesystem more reliable. The number of after hours call-ins hasbeen reduced: in 1997 there were 42 call-ins; in 2004 therewere 6 call-ins; in the 4 operating months of 2005 there wasonly 1 call-in, saving approximately $20,000 in labor costs peryear.

ACKNOWLEDGMENT

We gratefully acknowledge the support and collaboration ofD. K. Cummings.

REFERENCE

[1] K.L. Holtrop, P.M. Anderson, and P.S. Mauzey, �Improvements to thecryogenic control system on DIII-D,� Proc. 20th IEEE/NPSS Symp. onFusion Engineering, San Diego, California (Institute of Electrical andElectronic Engineers, Inc., Piscataway, New Jersey, 2003), p. 401.